In the work of electricians, a common task is to connect a three-phase motor to a single-phase network. To accomplish this, at first glance, a difficult task without the help of additional devices is difficult. Devices that allow a three-phase motor to operate in a 220 V network are various phase-shifting elements. Of their variety, a container is most often chosen for these purposes. You can choose the right capacitor for a three-phase motor using diagrams and simple formulas.
Content
- Features of a three-phase motor
- The need for phase-shifting elements
- Capacitor circuits
- Calculation of the required capacity
- Choosing a wiring diagram
- Power-up faults
Features of a three-phase motor
Asynchronous electric motors with three stator windings prevail in various branches of agriculture. They are used to drive ventilation devices, manure removal, feed preparation, and water supply. The popularity of such motors is due to a number of advantages:
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simplicity of structure; - reliability in work;
- when connected in normal mode, expensive and scarce devices are not used;
- the number of technical services is small.
You can try to connect a three-phase motor to 220, knowing the differences in the winding connection schemes. The number of phases for which the motor is designed can be determined by the number of terminals in its terminal box: a three-phase one will have 6 terminals, and a single-phase one has two or four.
The three-phase motor windings are connected according to an established pattern called "star" or "delta". Each of them has its own advantages and disadvantages. When connected to a star, the ends of the windings are connected. In the terminal box, this connection diagram will be displayed using two jumpers between the terminals marked "C6", "C4", "C5". If the motor windings are connected in a triangle, then a beginning is attached to each end. Three jumpers will be used in the terminal box, which will connect terminals "C1" and "C6", "C2" and "C4", "C3" and "C5".
The need for phase-shifting elements
When a three-phase electric motor is connected to a 220 V network, the starting torque does not occur. Therefore, it becomes necessary to connect starting devices. They create a phase shift that allows the motor to start and run under load for a long time.
As phase-shifting elements can be used:
- resistance;
- inductance;
- capacity.
Due to the connection of a three-phase motor through a capacitor, the shaft starts to rotate when voltage is applied. Connecting the tank guarantees the motor not only starting, but also holding the load for a long time.
Capacitor circuits
It is possible to connect a three-phase electric motor to a 220 V network only after studying the winding connection diagram and the purpose of the device that it will operate.
The connection of the capacitor to the motor windings must be carried out, observing some rules. A three-phase motor is connected to a single-phase network using one of two standard schemes: "star" or "delta".
In motors of medium and high power, two capacities are required - working and starting. The working capacitor Cp is necessary for the appearance of a circular field at the nominal operating mode. The starting capacitor Cn is needed to create a circular field when starting with a rated load on the shaft.
Connection procedure for "star":
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Connect the ends of the windings in a star in the terminal box (put jumpers between terminals C6, C4, C5). - Connect the container to the beginning of any two coils (C1, C2 or C2, C3 or C3, C1).
- The voltage of 220 V must be applied to the beginning of the free winding and the one that is connected to the capacitor. Since there is no polarity in alternating current, which particular motor coil to apply voltage to, there is no difference.
Connection procedure for the "delta" circuit:
- Connect the terminals of the motor coils in the terminal box by installing three jumpers between terminals C1 and C6, C2 and C4, C3 and C5.
- Connect capacitors to the beginning and end of one phase (C1, C4 or C2, C5 or C3, C6).
- Bring zero to the capacitor-free jumper terminal, and the phase to any other terminal.
To change the direction of rotation of the shaft, either voltage or capacitors must be connected to another phase of the motor.
Calculation of the required capacity
When choosing a capacitor, it is necessary to prevent a situation in which the phase current exceeds its nominal value. Therefore, the calculations must be approached very carefully - incorrect results can lead not only to breakdown of the capacitor, but also to burnout of the motor windings.
In practice, to start low-power motors, a simplified selection is used based on the considerations that for every 100 W of motor power, 7 μF capacitance is needed when connected in a triangle. When the winding is connected to a star, this value is halved. If a three-phase motor with a power of 1 kW is connected to a single-phase network, then a capacitor with a charge of 70-72 μF is needed when the windings are connected with a triangle, and 36 μF in the case of a star connection.
The calculation of the required capacity value for work is carried out according to the formulas.
With a star connection:
Wed = 2800 I / U
If the windings form a triangle:
Wed = 4800 I / U
I is the rated current of the motor. If for some reason its value is unknown, for the calculation it is necessary to use the formula:
I = P / (3 U)
In this case, U = 220 V when connected with a star, U = 380 V - with a triangle.
P is the power measured in watts.
When starting the engine with a significant load on the shaft in parallel with the working capacity, it is necessary to turn on the starting one.
Its value is calculated by the formula:
Cn = (2.5 ÷ 3.0) Cp
The starting capacity should exceed the working capacity by 2.5 - 3 times.
The correct choice of voltage value for the capacitor is very important. This parameter, like the capacity, affects the price and dimensions of the device. If the mains voltage is higher than the rated value of the capacitor, the starter will fail.
But it is also not worth using equipment with overvoltage. After all, this will lead to an ineffective increase in the size of the capacitor bank.
The optimum is the value of the capacitor voltage 1.15 times higher than the value of the mains voltage: Uk = 1.15 U s.
Very often, when a motor with three windings is turned on in a single-phase network, capacitors of the KGB-MN or BGT type (heat-resistant) are used. They are made of paper. The metal case is completely sealed. Has a rectangular appearance. It must be borne in mind that the permissible voltage and capacitance values indicated on the device are indicated for direct current. Therefore, when operating on alternating current, it is necessary to reduce the capacitor voltage indicators by 2 times.
Choosing a wiring diagram
The windings of the same motor can be connected either star or delta. You need to choose a connection scheme according to the load. If a three-phase motor in a single-phase network will drive any low-power mechanism, then you can choose a "star" connection scheme. In this case, the operating current will be small, but the dimensions and price of the capacitor bank will be significantly reduced.
In case of heavy load during operation or at the moment of starting, the motor windings must be connected according to the "delta" scheme. This will provide sufficient current for continuous operation. The disadvantages include the significant price and size of the capacitors.
Power-up faults
If, after connecting the capacitors and applying voltage, the motor hums but does not start, the reasons can be varied:
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insufficient capacitance of the capacitor (the ammeter will record a current exceeding the permissible value); - damage to the jumper or supply wire;
- wrong connection;
- energizing the wrong winding.
Loud unpleasant noise when the motor is turned on and the shaft rotates indicates an exceeded capacitor capacity.
It will be nice to work a three-phase motor in a single-phase network. The only drawback will be the power it develops - not 100%, but 60-80% of the nominal. If the capacity is used only for starting, then the net power of the motor will not exceed 60% of its rated power.
